1. Field of the Invention
The present invention relates to a self-refresh apparatus for a semiconductor memory device and more particularly, to a self-refresh apparatus for self-refreshing a memory cells of a semiconductor memory device such as a dynamic random access memory (DRAM).
2. Description of the Conventional Art
A typical DRAM configuration is a one-transistor memory device configuration in which each one-bit memory cell is composed of a MOS transistor and a capacitor. In this case, the data in memory cells will disappear as time passes because of leakaging currents. Accordingly, the DRAM requires a refresh operation to be carried out at predetermined intervals, to read stored data from the memory and to rewrite the data to the memory.
FIG. 1 shows a block diagram for performing a conventional self-refresh operation. As shown in FIG. 1, when a self-refresh command (selfref command) is inputted to a self-refresh state control unit (1), the self-refresh state control unit (1) outputs a self-refresh state signal (sref) to a ring oscillator (2).
The ring oscillator (2) is operated by the self-refresh state signal (sref) and produces a pulse signal (1 .mu.s Period) having 1 .mu.s period. The pulse signal (1 .mu.s Period) is inputted to a frequency divider (3) and is converted into a pulse signal having 16 .mu.s period.
Then, a self-refresh request control unit (4) receives a pulse signal (f16 .mu.s) having a 16 .mu.s period outputted from the frequency divider (3) and the self-refresh state signal (sref) produced by the self-refresh state control unit (1), and then produces a self-refresh request signal (selfreq) every 16 .mu.s.
The self-refresh request signal (selfreq) is inputted to an internal row active control unit (5), refreshes cells corresponding to an internal address and increases the number of address by means of an internal address counter.
FIGS. 2a and 2b show timing charts for explaining the self-refresh operation of FIG. 1. FIG. 2a shows a situation where the cell data are destroyed owing to the decrease of the retention time of cell data in accordance with variations in temperature, voltage, manufacturing process and the like. As shown in FIG. 2a, a refresh time of following cells becomes longer than the retention time of data, thereby causing data in the cells to be destroyed.
FIG. 2b shows a situation where the retention time of cell data is increased. In the conventional self-refresh mode as shown in FIG. 1, there are cells unnecessarily refreshed, thereby causing the unnecessary power consumption.